The Mechanism of Excimer Laser-Induced Amorphization of Ultra-Thin Si Films
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(a-Si:H) on an oxide surface at high energy densities can lead to complete amorphization (i.e., conversion to four-fold coordinated amorphous Si) of the film [1, 21. The observed phenomenon is remarkable in that, due to the nonparticipating and inert nature of the oxide interface [3, 41 (i.e., it does not initiate growth of solid Si nor does it well catalyze the heterogeneous nucleation of crystals), the thermal and kinetic path through which formation of amorphous Si proceeds in these experiments is
expected to be clearly distinct from the much-investigated interfacial amorphization oj the surface region of Si wafers observed in pulsed laser-induced melting and regrowth experiments [5]. The situation, on the other hand, is physically more similar to rapid quenching and supercooling of isolated liquid droplets [6, 71 and is intimately related to deep supercooling of liquid Si films on SiO 2 [3, 4]. It is now 'possible to envision a number of transformation scenarios, ranging from schemes based on discontinuous first-order transitions to those which approach continuous transitions. Fundamentally, the situation warrants our attention because the involved cooling rates of 1-Si represent the highest quenching rates (>1010 K/sec) that are encountered in rapid solidification experiments to date. When coupled with the beneficial simplicity of the system being elemental and the experimental flexibility inherent in thin film methods, the situation presents materials scientists with a unique opportunity to probe experimentally into the previously uncharted kinetic territory of phase transformations in a rigorous manner.
725 Mat. Res. Soc. Symp. Proc. Vol. 321. ©1994 Materials Research Society
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Figure 1. Planar view bright field TEM images of laser irradiated Si films at various energy densities. The first sequence ((a) to (d)). the second sequence ((e) to (h)), and the third sequence ((i) to (I))correspond to the series in which the temperature of the substrates were at room temperature. 3d 0°C, and 4250C, 2respectively. The2 following are the~specific energy clensities: mJ/cml, (e) 71 mJ/cm , (f) 11Y (a) 150 2mJ/cm', (b) 1642 mJ/cm , (c) 1812 mJ/cm , (d) 192 , (h) 138 mJ/cm . (i) 119 mJ/cm2 , (J) 125 mJ/cm2 , (k) 139 nkJ/cm nmJ/cm , (g) 132 nj/cm 2 and (a)143 mJ/cm . In this paper, we report on an experimental investigation, which is conducted in
order to first characterize the conditions that lead to amorphization of thin Si films and second to probe into the details of the phase transformation mechanism behind ELA of Si films on SiO2 . In particular, we elaborate on the microstructures of nearly-
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amorphized Si films, and show that the observed heterogeneous microstructure suggests that amorphization occurs via discontinuous first-order transitions in which the interfacial amorphization is triggered by solids that are nucleated within deeply supercooled liquid Si. EXPERIMENTAL METHOD
Single-pulse irradiation experiments were conducted on non-hydrogenated LPCVD a-Si films (500
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